Graduation date: 1990Dispersal and colonization of young potted apple\ud trees by the spider-mite predators Metaseiulus\ud occidentalis Nesbitt and Typhlodromus pyri Scheuten\ud were studied in the Hood River Valley of Oregon.\ud Experimental mini-orchards of 10 four-year old potted\ud apple trees were established at distances of 0, 10, and\ud 100 m downwind from commercial apple orchards. Mini-orchards\ud were inoculated with Panonychus ulmi and Tetranychus urticae to provide a prey resource for\ud predatory mites. The commercial orchards harbored\ud either M. occidentalis or T. pyri, or both.\ud Emigration from the source orchards was estimated\ud by the number of adult females found in mini-orchards\ud located 0 m downwind. Immigration was measured by the\ud initial presence of adult females in mini-orchards at\ud 10 and 100 m. Colonization was interpreted as the\ud presence of predatory mite eggs or nymphal stages in\ud samples from the 10 and 100 m mini-orchards. Leaf\ud samples were taken either biweekly (1987) or weekly\ud (1988).\ud Metaseiulus occidentalis and T. pyri both\ud emigrated from the source orchard trees to the\ud experimental mini-orchards 0 m downwind. Typhlodromus\ud pyri rarely immigrated to the mini-orchards 10 m\ud downwind, while M. occidentalis reached the potted\ud trees at 10 m soon after the mini-orchards were\ud established. Metaseiulus occidentalis also immigrated\ud to the experimental mini-orchards 100 m downwind,\ud whereas T. pyri did not. In all cases, immigration of\ud M. occidentalis to the experimental mini-orchards was\ud more frequent than for T. pyri. Also at all sites, M.\ud occidentalis colonized the young apple trees at a\ud faster rate than did T. pyri
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